Conventional mechanical refrigeration systems provide gaseous refrigerant to a compressor. The compressor discharges the gaseous refrigerant under substantial pressure to a condenser where the compressed gas is cooled and condensed into a liquid. The liquid refrigerant flows from the condenser to a receiver where the liquid is accumulated. The liquid then travels from the receiver to an expansion valve where the pressure upon the liquid is decreased. The refrigerant, now at low temperature and pressure, flows into an evaporator. In the evaporator, the refrigerant absorbs heat from the space to be refrigerated. This heat absorption transforms the liquid into a gas. The gaseous refrigerant is then drawn through a suction line to the compressor where it is again compressed and the above-described cycle repeated.
Conventional refrigeration equipment employs various arrangements for periodic defrosting of the evaporator in order to maintain the evaporator free from the accumulation of ice or frost. Such devices typically incorporate a bypass line so that hot gas refrigerant discharged from the compressor bypasses the condenser and travels directly to the receiver. The hot gas exits the receiver, bypasses the expansion valve, and enters the evaporator. The hot gas, as it travels through the evaporator, melts the accumulation of frost or ice on the evaporator coil. A single conduit is commonly used during defrost and refrigeration, i.e., a single conduit carries liquid during refrigeration and hot gas during defrost.
In more detail, a typical hot gas defrost system closes a valve at the inlet to the condenser. This forces all the hot gaseous refrigerant produced by the compressor to bypass the condenser and to flow directly into the receiver. The pressure of the hot gas entering the receiver forces all the liquid out of the receiver and into a conduit leading towards the evaporator. The pressure provided by the compressor then forces the hot gas out of the receiver and into the conduit towards the evaporator. The evaporator generally contains two inlets, a liquid inlet used during refrigeration and a hot gas inlet used during defrost. During defrost, a valve on the liquid inlet to the evaporator closes and a valve on the hot gas inlet opens. This forces the hot gas to flow through the hot gas valve and into the hot gas inlet of the evaporator. The hot gas then traverses the distributor and the evaporator coil. The heat from the hot gas is conducted and transferred to the evaporator coil. This warms the coil and, in turn, melts the ice or frost. This process also condenses the gaseous refrigerant into a liquid. The liquid refrigerant then flows through the evaporator outlet and into a suction line to the compressor. During defrost, a valve closes on the suction line and forces the liquid refrigerant into a branch line. The branch line may contain a holdback valve, accumulator, or other such device to limit the pressure at the compressor inlet. Limiting the pressure at the compressor inlet is necessary to prevent the compressor inlet pressure from overloading the compressor. The liquid in the branch valve is then reheated into a gas. The gas is then returned to the compressor and the defrost cycle can be repeated.